Spectrum sensing is needed in cognitive radios to find empty slots in the radio spectrum which can subsequently be used in an opportunistic manner. The cognitive radio must be able to detect a primary user at a low power level and avoid causing interference to it. Generally it is assumed to use the actual mobile terminals operating within the network as the cognitive radios that also sense the spectrum quality. There are various proposals by which cognitive radios may find the spectrum that is available for use, and these teachings proceed from the assumption that the cognitive radios have indeed found some available spectrum that they can use without interfering with the existing networks (which may be hierarchical or ad-hoc) that are specifically authorized to use a portion of the universe of radio spectrum. Development of cognitive radio systems is at an early stage, and come cognitive radio systems may have a specific spectrum band allocated and may even have some central node. Regardless of whether radio spectrum bands are allocated only to the primary network systems or additionally to the cognitive system, the cognitive radio terminals themselves use the frequencies within a spectrum band opportunistically.
Term the radios operating in those primary networks with frequency resources assigned to users as the primary users, and term the cognitive radios that use frequency resources on an opportunistic basis as the secondary users. The secondary users are obliged to avoid the frequencies that the primary users are using, which is by definition why they are cognitive radios; they must find the spectrum that is available for use. Among the secondary users the spectrum must be used in such a way that the transmission of one secondary system (e.g., a pair of cognitive radio terminals communicating with one another) is minimally interfering the transmission of another secondary system. All of these secondary users are equal in status as far as spectrum usage is concerned; they compete against each other to find a part of spectrum where to operate.
There arise instances where the spectrum might be so crowded that a secondary user is incapable of finding an empty spectrum slot for its use. This may occur for example if the primary users occupy a particularly large proportion of their network's spectrum leaving little to cognitive users to exploit, or if there are a large number of simultaneous cognitive users so that the competition for the exploitable spectrum is tight. Since the cognitive radios/users in such a scenario are equal in rank and by definition for a cognitive system there is no hierarchy by which to allocate the scarce exploitable spectrum, this leads to a problem. How does one cognitive radio get an opportunity to transmit, and particularly in a manner that does not interfere with other cognitive users who may or may not already be communicating? Without some solution the result would appear to be that there would be multiple interferences among the equal secondary users, wasting the scarce spectrum available and exacerbating the problem. These teachings are directed to resolving that problem.